This invention relates to a process for gasifying coal, more particularly it relates to a process for gasifying coal using a two-stage entrained flow gasifier with high gasification efficiency and excellent operation controllability.
Processes for gasifying coal with high efficiency to give raw materials for chemicals, fuel for industrial use and town gas are now under development.
Entrained flow coal gasification wherein the gasification temperature is higher than the melting point of coal ash has the following features and is expected to be employed widely as coal gasification process:
(a) Gasification rate of carbon can be increased. PA1 (b) Since tar can be decomposed completely, troubles caused by tar can be prevented. PA1 (c) Since coal ash can be taken out in the form of molten slag, it can be disposed as it is without causing much troubles in environmental pollution. PA1 (i) A process wherein coal is subjected to pyrolysis, and produced char is subjected to combustion or partial combustion, while using the heat generated as heat source for the pyrolysis. (Bi-Gas process, e.g., "Recent Development in High Pressure, Entrained Flow, Slagging Gasification of Coal" by R. K. Young in 8th International Conference on Coal Gasification, Liquefaction & Conversion to Electricity: University of Pittsburgh, Pittsburgh, PA., August 4-6, 1981). PA1 (ii) A process wherein coal is supplied not only to a pyrolysis zone but also to a combustion zone of char and is gasified together with char. (Combustion Engineering Process, e.g., U.S. Pat. No. 4,168,956).
The entrained flow coal gasification processes can be divided into two categories, one of which comprises injecting a gasifying agent (oxygen or air and steam) and coal through a single feed burner to a gasifier to produce gases rich in H.sub.2 and CO by partial combustion (a one-stage gasification process) and the other of which comprises supplying coal or char which is a pyrolysis product of coal separately from two feed burners to produce gases rich in H.sub.2, CO and CH.sub.4 by pyrolysis and partial combustion (a two-stage gasification process).
There are known the following two main processes in the two-stage gasification process:
But even employing the one-stage or two-stage entrained flow gasification processes mentioned above, coal is not gasified completely in a gasifier by one pass, so that unreacted carbon particles are present in particles flying out of the gasifier (such particles are called "char") and the gasification efficiency cannot be improved unless the char is recovered and recycled to the gasifier.
Typical gasification reactions of coal can be represented by the following equations: EQU Coal.fwdarw.Char(C)+CH.sub.4 +H.sub.2 +CO (1) EQU C+O.sub.2 .fwdarw.CO.sub.2 ( 2) EQU C+CO.sub.2 .fwdarw.2 CO (3) EQU C+H.sub.2 O.fwdarw.H.sub.2 +CO (4) EQU CO+H.sub.2 O.revreaction.CO.sub.2 +H.sub.2 ( 5)
The pyrolysis reaction (1) and the shift reaction (5) take place relatively rapidly and the combustion reaction (2) is completed in very short time. But the reactions (3) and (4) are slow in reaction rate compared with the rest of the reactions and take much time for gasification. Therefore the improvement in gasification efficiency depends on how to make the reactions (3) or (4) faster. The reaction rate in the reaction (3) or (4) is influenced by the reaction temperature, partial pressures of gasifying agents, properties of coal particles, etc. According to the gasification processes mentioned above, optimum conditions are not always employed, so that char is discharged from a gasifier.
On the other hand, recycle of char gives the following defects in operation of gasifying apparatus. Since char is recovered, in general, in afterflow of the gasifier, that is, in a place wherein the pressure is lower than that of the gasifier (e.g. a cyclone), char tends to flow reversely to the lower pressure side in the recycling system. Thus, it is necessary to install devices or apparatus which are resistant to the gas flow such as a char feeder, valves, a particle packing layer, etc., which results in making the controlling system of the char recycle complicated and making operating characteristics of gasifier worse. Particularly in the case of the Bi-Gas process mentioned above (i), since the recycle of char is a premise and the gasification temperature is maintained by combustion heat of char, reduction in flow rate of char in the recycle system or no flow of char lowers the temperature at the combustion zone, solidifies coal ash and closes the flowing down passage of slag, whereas the temperature at the pyrolysis zone is lowered, tar is produced and coking troubles are caused at the afterflow of gasifier, which results in causing stoppage of operation of the gasifier. Further, in the Bi-Gas process, when steam which is a gasifying agent is used for transporting char, control of the recycle system becomes more complicated since both the gasification conditions and the transport conditions should be satisfied at the same time.
The Combustion Engineering process mentioned above (ii) is improved in operation of the gasifier compared with the Bi-Gas process since coal is supplied from upper and lower portions separately and the gasification temperature can be maintained even the supply of char becomes unstable. But the gasification conditions are not optimum and char is still recycled.
As mentioned above, there is a fatal defect in the entrained flow gasification that improvement of gasification efficiency can only be attained by recycling char.